A twisted cable used as a conductor for aerial transmission line is required to have certain lightness and low thermal expansion coefficient because small slack is preferable in practice.
In general, such a twisted cable comprises a core having high physical strength and conducting metal wires such as aluminum wires twisted around the core.
In one of the prior arts, the core of the twisted cable is formed of invar wires having thermal expansion coefficient of 2.5.times.10.sup.-6 through 4.times.10.sup.-6 /.degree.C. lower than those of steel wires. In another prior art, the core of the twisted cable is formed of material including relatively light carbon fibers as disclosed in Japanese Patent Application Publication No. 40922/1981.
Although the Japanese Patent Application Publication No. 40922/1981 never refers to thermal expansion coefficient of carbon fibers which are used as material of the core, it is well known that the thermal expansion coeffcient of carbon fibers is equal to or lower than that of the invar wires. Thus, it is confirmed that the core formed of carbon fibers reinforced by resin has thermal expansion coefficient not higher than 2.times.10.sup.-6.
It is supposed that the core including carbon fibers may be manufactured in the following manner.
A plurality of carbon fiber filaments having a diameter of 7 through 10 .mu.m are impregnated with resin and are twisted to form a carbon fiber twisted element. The thus producetd twisted element has a tape of polyester lapped thereon to form element lines.
The element lines may be used as the core of the twisted cable either in a straight manner or in a twisted manner after the impregnated resin is cured.
This is because the core formed of only carbon fibers has relatively low physical strength and is likely to to snap off as soon as undergoing bending stress unless the carbon fibers are cured with resin.
In general, an aerial transmission cable is subject to high temperature during its operation to thereby cause a problem. This problem can be solved by heightening thermal resistance of the resin used. Practically, the core can withstand a temperature of 240.degree. C. at most.
On the other hand, the aerial transmission cable has accidental insulation destruction due to lightning stroke and a subsequent alternate arc generated when reverse flashing runs from the transmission cable to ground. At the moment, a temperature of the core reaches 1000.degree. C. or possibly a few thousands degree C. for a very brief time because of the alternate arc. Thus, aluminum wires which are the conducting metal wires are often melted and the high heat sometimes reaches the core. But, since no resin can withstand a temperature higher than 1000.degree. C., the resin will burn out when the core is subject to such a high temperature.
Such being the case, the prior twisted cable comprising the core of carbon fibers reinforced by resin and the conducting metal wires such as aluminum wires twisted around the core will lose resin which serves to maintain the physical strength of carbon fibers which usually endure the arcing when the twisted cable is subject to the arc. Thus, since there occurs a breakage of the twisted cable, which causes them to be lacking in its reliability.
On the other hand, the cable having a core including invar wires is heavy-weighted and has thermal expansion coefficient higher than that of the core including carbon fibers at high temperature. Therefore, the twisted cable has a large amount of slack in practice.